Rare metals and platinum-group metals, for example: cobalt (Co), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), platinum (Pt), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), zinc (Zn), yttrium (Y), zirconium (Zr), niobium (Nb), silver (Ag), cadmium (Cd), lanthanum (La), cerium (Ce), neodymium (Nd), europium (Eu), terbium (Tb), gold (Au), mercury (Hg), and uranium (U), are not only used as catalyst, but also used in a wide range of applications such as hydrogen storage alloys, battery, and magnet. Japan heavily depends on imports for these metals, so, in view of stable supply of resources and environmental protection, recycle of these rare metals and platinum-group metals has become a major issue.
At present, there is a method to efficiently recover useful metal such as rare metals from defective goods, waste, and wastewater from factories, the method including the steps of: making the waste into an aqueous solution after acid treatment; and extracting the rare metals and so on from the aqueous solution. As the extractant for the solvent extraction, various extractants are on sale and used. However, the aqueous solution produced by the acid treatment has various pH and concentration conditions; in addition to this, various kinds of metals are mixed. Therefore, to extract metals selectively from the aqueous solution under these conditions, multi-step extraction using combined extractant is conventionally performed; which is extremely expensive and takes longer period of time.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 11-179104    Patent Document 2: JP-A No. 11-199581    Patent Document 3: JP-A No. 2000-107505    Patent Document 4: JP-A No. 2000-178271    Patent Document 5: JP-A No. 2001-149884    Non-Patent Document 1: Chem. Lett., 6, 2001. Talanta 62, 337, 2004